Tracing high energy radiation with molecular lines near deeply embedded protostars

Aims. The aim is to probe high energy radiation emitted by deeply embedded protostars. Methods. Submillimeter lines of CN, NO, CO+ and SO+, and upper limits on SH+ and N2O are observed with the James Clerk Maxwell Telescope in two high-mass and up to nine low-mass young stellar objects and compared with chemical models. Results. Constant fractional abundances derived from radiative transfer modeling of the line strengths are x( CN) approximate to a few x 10(-11) - 10(-8), x(NO) approximate to 10(-9) - 10(-8) and x(CO+) approximate to 10(-12) - 10(-10). SO+ has abundances of a few x 10(-11) in the high-mass objects and upper limits of approximate to 10(-12) - 10(-11) in the low-mass sources. All abundances are up to 1 - 2 orders of magnitude higher if the molecular emission is assumed to originate mainly from the inner region ( less than or similar to 1000 AU) of the envelope. For high-mass sources, the CN, SO+ and CO+ abundances and abundance ratios are best explained by an enhanced far-ultraviolet (FUV) field impacting gas at temperatures of a few hundred K. The observed column densities require that this region of enhanced FUV has scales comparable to the observing beam, such as in a geometry in which the enhanced FUV irradiates outflow walls. For low-mass sources, the required temperatures within the FUV models of T greater than or similar to 300 K are much higher than found in models, so that an X-ray enhanced region close to the protostar ( r less than or similar to 500 AU) is more plausible. Gas-phase chemical models produce more NO than observed, suggesting an additional reduction mechanism not included in current models. Conclusions. The observed CN, CO+ and SO+ abundances can be explained with either enhanced X-rays or FUV fields from the central source. High-mass sources likely have low opacity regions that allow the FUV photons to reach large distances from the central source. X-rays are suggested to be more effective than FUV fields in the low-mass sources. The observed abundances imply X-ray fluxes for the Class 0 objects of L-X approximate to 10(29) - 10(31) erg s(-1), comparable to those observed from low-mass Class I protostars. Spatially resolved data are needed to clearly distinguish the effects of FUV and X-rays for individual species.